1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed using a thin film transistor (hereinafter referred to as TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic appliance in which an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element is mounted as its component.
Note that the semiconductor device in this specification refers to all the devices which can operate by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor formed using two or more kinds of atoms bonded together. In general, metal oxides become insulators. However, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are metal oxides which have semiconductor characteristics. A thin film transistor in which a transparent semiconductor layer which is formed using such a metal oxide serves as a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m is a natural number), which is a homologous compound, is a known material (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (Patent Document 5 and Non-Patent Documents 5 and 6).
Further, attention has been drawn to a technique for manufacturing a thin film transistor using an oxide semiconductor, and applying the thin film transistor to an electronic device or an optical device. For example, Patent Document 6 and Patent Document 7 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a thin film transistor is used as a switching element or the like of an image display device.